1. Field of the Invention
The present invention relates to a contactless (wireless) method of supplying power that disposes a transmitting coil and a receiving coil in close proximity to magnetically couple the coils to supply power from the transmitting coil to the receiving coil by magnetic induction, and in particular to a contactless method of supplying power that detects foreign objects on the power supply stand.
2. Description of the Related Art
A contactless method of supplying power that places a portable device housing a receiving coil on a power supply stand (pad, platform, stage, cradle) housing a transmitting coil and transmits power from the transmitting coil to the receiving coil has been developed. (Refer to Japanese Laid-Open Patent Publication 2008-17562.)
In the contactless method of supplying power of the related disclosure, a charging pad serves as the power supply stand, a battery powered device serves as the portable device, and power is transmitted from the charging pad to the battery powered device to charge a battery internal to the battery powered device. For contactless charging, the portable device receiving coil is put in close proximity with the power supply stand transmitting coil to supply power from the transmitting coil to the receiving coil. The internal battery is charged by power induced in the receiving coil. In this method of supplying power, power can be transmitted to the portable device in a contactless manner and there is no need to connect the portable device to the power supply stand via connectors.
This method of supplying power has the drawback that if a metal foreign object such as a paper clip is on the power supply stand when it is supplying power, current will be induced in the foreign object, which will generate heat due to Joule-heating. Further, since wasted power is consumed by current induced in the foreign object, this method also has the drawback that power cannot be supplied from the power supply stand to the portable device in an efficient manner.
To eliminate these drawbacks, the charging pad cited in JP 2008-17562 has many temperature sensors disposed in an x-y array on its upper surface to detect foreign objects. The temperature sensors detect heat generated by a foreign object on the charging pad. If alternating current (AC) power is supplied to the transmitting coil of this charging pad with a metal foreign object on its upper surface, heat will be generated by current induced in the foreign object and that heat will be detected by a nearby temperature sensor.
Since the charging pad described above requires temperature sensors for detection and decision circuitry to judge the presence of foreign objects from temperature detected by the temperature sensors, it has the drawbacks of complex circuit structure (for foreign object detection) and high manufacturing cost. It also has drawbacks such as the inability to stably and reliably detect a foreign object positioned at a distance from the nearest temperature sensor, and delayed detection (resulting in detrimental conditions such as excessive heat generated by the foreign object). Further, since this charging pad judges the presence of a foreign object by detecting its heat generation with temperature sensors, it also has the drawback that the foreign object cannot be safely detected without heating it (possibly to an excessive temperature).
The present invention was developed with the object of further eliminating the drawbacks described above. Thus, it is an object of the present invention to provide a contactless method of supplying power that can quickly, stably, and reliably detect foreign objects with a simple circuit structure and low manufacturing cost. Further, it is another object of the present invention to provide a contactless method of supplying power that can safely detect foreign object presence.